1. Field of the Invention
The present invention relates to a group III nitride compound semiconductor device and a method of producing the same.
The present application is based on Japanese Patent Applications No. Hei. 11-130475 and 11-266499, which are incorporated herein by reference.
2. Description of the Related Art
A group III nitride compound semiconductor device such as a blue light-emitting device, or the like, was produced by the steps of: growing a buffer layer of AlxGa1-xN (0≦X≦1) on a sapphire substrate by a metal organic chemical vapor deposition method (abbreviated as “MOCVD method” in this specification); and further growing a group III nitride compound semiconductor layer on the buffer layer by the same MOCVD method.
In the MOCVD method, an ammonia gas and gases of group III metal alkyl compounds such as trimethylaluminum (TMA), trimethylgallium (TMG) and trimethylindium (TMI) are supplied onto a substrate heated to an appropriate temperature and are thermally decomposed so that a film of a desired crystal is formed on the substrate. On this occasion, metal organic compounds such as TMA, and so on, which serve as raw-material gases for forming the buffer layer, are expensive. This was a factor of increasing the cost of the group III nitride compound semiconductor device.
If the buffer layer of AlxGa1-xN (0≦X≦1) is formed by a method other than the MOCVD method, the use of metal organic compounds such as TMA, TMG, and so on, can be avoided. For example, Japanese Patent Publication No. Hei. 5-86646 has proposed a method in which a buffer layer is formed by a high-frequency sputtering method; group III metal organic compounds are supplied to the buffer layer after the buffer layer is heated (to a temperature of from 800° C. to 1000° C.) in an atmosphere containing an ammonia gas (ammonia and nitrogen in an embodiment); and then the group III metal organic compounds are decomposed on a heated substrate so that a nitride film thereof is vapor-grown and AlxGa1-xN (0≦X≦1) of the same composition is grown on the buffer layer. Raw materials used for forming the buffer layer of AlxGa1-xN (0≦X≦1) by the high-frequency sputtering method are high-purity metallic aluminum and metallic gallium. A mixture gas of argon and nitrogen with these metals as targets is used as a sputter gas. In this case, all the raw materials are inexpensive. Hence, the cost of the device can be reduced compared with the case where expensive metal organic compounds are used as raw materials for forming the buffer layer by the MOCVD method.
Although the inventors of the present invention tried the method disclosed in Japanese Patent Publication No. Hei. 5-86646, the crystallinity of the group III nitride compound semiconductor layer which is formed by the MOCVD method so as to be grown on the buffer layer of AlxGa1-xN (0≦X≦1) formed by the high-frequency sputtering method did not satisfy the inventors' requirements. That is, the crystallinity of the group III nitride compound semiconductor layer obtained in the aforementioned manner was inferior to the crystallinity of the group III nitride compound semiconductor layer which is formed by the MOCVD method so as to be grown on the buffer layer of AlxGa1-xN (0≦X≦1) formed by the MOCVD method.